Sofia, Bulgaria — While Bulgarian émigré Georgi Markov walked over Waterloo Bridge in London on Sept. 7, 1978, a passerby bumped into the well-known critic of his native government. A stinging pain shot through Mr. Markov's calf, and four days later he was dead.

Investigators initially thought an assassin, hired by the communist regime in Bulgaria, jabbed him with a poison-tipped umbrella. But later reports suggested a spring-loaded pen, probably KGB-designed, had fired a ricin-tipped pellet into his leg.

Today much of the Markov murder remains shrouded in mystery. The case, however, is just one of many unsolved mysteries spurring intense debate in Eastern Europe between critics and defenders of the communist system.

Though the days of Soviet control are but a distant memory, revelations about who was once a spy or informant continue to rock the region. Many communist-era officials remain in power and continue to hold onto a number of secrets about the past, not only to protect themselves and their allies, but the reputation of the former dictatorships.

Earlier this month, Milan Kundera, the Czech author of "The Unbearable Lightness of Being" and a noted critic of communism, was accused of reporting a Western intelligence agent to authorities 58 years ago. Mr. Kundera emerged from 23 years of public silence to deny the charges.

For Markov's brother, Nikola Markov, finding the truth about Georgi's murder is no longer personal, he says, it's now about enabling broader social justice.

"My brother's murder is not a criminal case, but a political case," says Nikola. "I'm not looking for the killer, but [the person] who ordered and organized it. And who was guilty? The system. I want to show the world what the communist system really was."

Scotland Yard reopened its Markov investigation earlier this year. But even 18 years after the fall of the communist regime in Bulgaria, Markov's supporters charge that the nation's security service consistently block or undermine their attempts to find more information.

Most recently, the Bulgarian government's chief investigator dismissed the death as due to a British "medical blunder."

Bulgarian investigative journalist Hristo Hristov has researched the case since 1992, when he was a young court reporter covering the trial of Bulgaria's intelligence chief, who was sentenced to 11 months in jail for destroying key Markov-related files. The case piqued his curiosity about Markov and his prodigious body of work.

With Bulgaria's first open-archive law in 1997, Mr. Hristov determined to take a deep look into the case, despite rumors that all files about Markov had been destroyed.

This past summer, after a three-year court battle, he finally extracted 97 secret files that detail the KGB role in Markov's murder. The files comprise the heart of his second book on the Markov case, which he just published last month.

It hasn't been easy for Hristov, though. Even though the Bulgarian government posthumously decorated Markov in 2000 with the nation's highest honor for his contribution to the nation's literature and "confrontation" with the communist regime, Hristov's apartment has been ransacked three times as a result of his research into the Markov case. He's since relocated his most important files to his newspaper's office.

Others who continue to dig into government affairs, face similar communist regime-style attacks.

Just last month, the editor of a news website that probes security-service activities and high-level corruption was beaten unconscious by three attackers wielding iron pipes.

During more difficult moments, Hristov says he often turns to Markov for inspiration. He says much work remains, not only regarding Markov, but other archived mysteries that Bulgarians could never get answers to, let alone ask about, before the fall of communism.

"I keep asking myself, what if Markov were afraid? He wouldn't have published," says Hristov. "But he did publish, so his stories are an example for us to follow today."

Emboldened by Hristov's findings, Nikola Markov, who now lives outside Milan, issued an ultimatum to the Bulgarian authorities last month to replace the lead investigator, reopen the case, and cooperate with Scotland Yard, or he will take the case to the European Court of Human Rights in Strasbourg, France.

Mr. Markov says he's still awaiting an official response.

"Nobody else had the guts to go into the security archives, to find some proof," he says. "I thank Hristo one hundred times, for if it weren't for him, the case would've been forgotten."

Even in death, then, Georgi Markov remains a thorn in the government's side. He had burst upon the literary scene in 1962 with a novel, "Men," which won the top annual prize from the Union of Bulgarian Writers. Yet his pen soon became more pointed with several plays that communist censors banned.

Sentenced in absentia to six years and six months in jail, in 1975 he began a regular BBC series that critiqued life back in Bulgaria. It so infuriated the authorities in Sofia, that Hristov's research suggests they complained to Moscow about both the damage he was doing to Bulgaria's international image and its potential to incite Bulgarians illegally listening to his broadcasts.

"While other émigrés just swore at the system, he was analyzing it. He was uniquely brave," says Hristov.